Newer mobile devices demand longer battery life, increased functionality, and smaller footprints. As one component utilized to achieve these goals, high-Q micromachined resonators provide an avenue to bring low power multi-modal filters, oscillators, and sensors on-chip. To further increase frequency selectivity (for filters), improve stability, and reduce power consumption in oscillators, the energy loss must be minimized to reach higher Q in resonators. Different sources of loss, including air damping, energy losses through the support, thermo-elastic damping, and internal friction losses must be considered.
Capacitive single crystalline resonators have repeatedly shown to yield very high Q (tens of thousands) as a result of low internal loss of silicon. However, these devices rarely show low motional impedances at high frequencies. In contrast, piezoelectric resonators, tend to show very small motional impedance and not as high of a quality factor.
Solidly mounted thickness-mode thin-film resonators may utilize acoustic reflectors to enhance their Q. For these devices, a Bragg reflector containing alternating high and low acoustic velocity materials is deposited in λ/4 (e.g., quarter wavelength) thicknesses under the resonant structure. These alternating layers create an acoustic mirror to reflect energy back to the resonate structure. However, due to the nature of the acoustic waves in a lateral extensional mode resonator, this approach is not applicable.
What is needed is a device for addressing the above, and related, issues.